1. Field of the Invention
This invention provides a porous catalyst structure with a high specific surface area that comprises a porous metallic substrate and a metallic catalyst layer on the surface of the substrate. More particularly, this invention provides a porous structure in which the metallic catalyst layer comprises a copper-zinc oxide (CuOZnO) and the catalyst substrate comprises a porous stainless steel, wherein the catalyst layer may optionally comprise Al2O3, ZrO2, or a combination thereof.
2. Descriptions of the Related Art
Catalysts are materials that help to improve the reaction rate when participating in a reaction, but will not be consumed in the reaction. For example, when used for exhaust gas treatment, the catalyst may allow deleterious gas molecules in the exhaust gas to be converted at a low temperature by decreasing the activation energy necessary for the gas decomposition reaction. Specifically, catalysts typically enable the desired reaction to proceed by decreasing the activation energy of the reaction, thus improving possibilities of occurrence of the reaction.
Presently, a wide variety of inorganic metallic oxides catalysts have been developed, which are generally produced by preparing granules of active metal oxides and then conglutinating the granules together. However, for typical reactions involving a solid catalyst, the catalyzed reaction usually occurs on the surface of the catalyst. Hence, if the reactants fail to enter the catalyst, the utilization factor of the catalyst will be degraded remarkably, which implies waste of both the reaction volume and materials. Moreover, most of the common metal oxides are all insulating materials with poor thermal conductivity, which restricts and adversely impacts the performance of the catalyst in a heated reaction.
To improve the performance of conventional solid catalysts, a cellular substrate with a high specific surface area has been adopted in some prior art solutions to support a catalyst to enlarge the contact area between the catalyst and the reactants. As an example, in a catalyst structure disclosed in JP5213681, a fiber-reinforced cellular ceramic substrate is formed by mixing a ceramic material with a high specific surface area, inorganic fibers and an inorganic adhesive together, molding or extruding the mixture, and then calcining the molded piece.
In the aforesaid cellular catalyst, the performance of the catalyst may be improved by increasing the specific surface area of the substrate, reducing the particle size of the catalyst components and improving the distribution pattern of the catalyst components. However, if more substrate (usually made of ceramic aluminum oxides) is added to increase the specific surface area, the ceramic material will become thicker without improving the contact area. For this reason, various improvements on the cellular substrate structure (e.g., the cellular form, density, wall thickness or the like) have been proposed, for example, in JP10-263416.
JP2003-245547 also discloses a cellular catalyst structure adapted for treating exhaust gas such as low-concentration carbon monoxide (CO). In this structure, the cellular substrate is also prepared by extruding a material with a high specific surface area and then calcining the extruded piece. The thickness of the partition walls between the individual penetrating holes, the length of the walls in the gas flow direction and the opening area ratio of the cellular structure are all controlled within specific ranges. Additionally, US 2006/0292340 discloses another cellular catalyst structure, in which a plurality of parallel through-holes partitioned by a plurality of partitions are formed on the substrate to increase the surface area of the substrate.
A commercialized product of cellular catalyst structures is the Diesel Three-Way Catalyst (DTWC) manufactured by PHITECS, in which a cellular catalyst substrate with 400 cells per square inch (CPSI) is used.
The cellular substrate structures enlarge the contact area between the active catalyst component distributed on the substrate surface and the reactants primarily by molding or extruding a ceramic material into the cellular form. Unfortunately, since such cellular catalyst substrates have huge volumes and weights, they suffer from some limitations in application. For example, it is difficult to weld the cellular catalyst to the reactor. Meanwhile, poor adhesion between the ceramic material and metallic catalysts (e.g., selected from a group consisting of Pd, Pt and other similar metals) can decrease the endurance of these catalysts.
Currently, another kind of cellular catalyst that uses a metallic material as a substrate has emerged in the market, for example, a metallic cellular catalyst produced by REEcat (web site: www.reecat.com). Generally, such metallic cellular catalysts are prepared in the following way. A wavy metal sheet is worked and rolled up into a cellular cylindraceous substrate. Then, a ceramic material (e.g., an aluminum oxide, a silicon oxide or the like) is coated onto the substrate through an immersion plating process to form a thin ceramic layer, and subsequently, a metal or metal oxide material functioning as an active catalyst is coated onto the thin ceramic layer. Finally, the assembly is subjected to a drying and calcining procedure to complete the cellular catalyst. This may improve the thermal conductivity of the catalyst and the gaseous kinetics of the reactants in the apertures of the catalyst, thus preventing pressure loss.
However, as being limited by the forming process, the aforementioned metallic cellular catalyst is formed thereon with a limited density of apertures (usually no more than 100 CPSI), which also imposes a limitation on the extent to which the surface area may be increased. Meanwhile, since such a structure is formed by processing and rolling up a wavy metal sheet into a cellular cylindraceous substrate, most of the reactants (e.g. deleterious gas) will reside within the full-through cylindraceous substrate, causing inadequate contact with the catalyst molecules during reaction. Furthermore, such a catalyst structure also has poor adhesion between the ceramic material and the metallic catalyst.
In view of the aforesaid problems, this invention provides a catalyst substrate with a high specific surface area, good thermal conductivity and stable adhesion without even occupying a large space to provide a porous catalyst structure demonstrating superior catalytic performance and applicability.